Investigations of amplitude and phase excitation profiles in femtosecond coherence spectroscopy

We present an effective linear response approach to pump-probe femtosecond coherence spectroscopy in the well-separated pulse limit. The treatment pre sented here is based on a displaced and squeezed state representation for t he nonstationary states induced by an ultrashort pump laser pulse or a chem ical reaction. The subsequent response of the system to a delayed probe pul se is modeled using closed form nonstationary linear response functions, va lid for a multimode vibronically coupled system at arbitrary temperature. W hen pump-probe signals are simulated using the linear response functions, w ith the mean nuclear positions and momenta obtained from a rigorous moment analysis of the pump induced (doorway) state, the signals are found to be i n excellent agreement with the conventional third-order response approach. The key advantages offered by the moment analysis-based linear response app roach include a clear physical interpretation of the amplitude and phase of oscillatory pump-probe signals, a dramatic improvement in computation time s, a direct connection between pump-probe signals and equilibrium absorptio n and dispersion lineshapes, and the ability to incorporate coherence assoc iated with rapid nonradiative surface crossing. We demonstrate these aspect s using numerical simulations, and also apply the present approach to the i nterpretation of experimental amplitude and phase measurements on reactive and nonreactive samples of the heme protein myoglobin. The role played by i nhomogeneous broadening in the observed amplitude and phase profiles is dis cussed in detail. We also investigate overtone signals in the context of re action driven coherent motion. (C) 2001 American Institute of Physics.